1. Field of the Invention
The present invention relates to a motor controller. More particularly, the present invention relates to a motor controller that adjusts the rotation velocity of a motor to a target rotation velocity.
2. Description of the Related Art
A Patent Document 1 cited below discloses a brushless motor controller. The controller comprises a first PWM (Pulse Width Modulation) circuit, a second PWM circuit and an AND circuit. The first PWM circuit repeatedly outputs an on-signal having a first pulse width at a first carrier frequency, with the first pulse width of the on-signal being modulated by the first PWM circuit in accordance with the target rotation velocity. The second PWM circuit repeatedly outputs an on-signal having a second pulse width at a second carrier frequency, with the second pulse width being different from the first pulse width, the second carrier frequency being lower than the first carrier frequency, and the second pulse width of the on-signal being modulated by the second PWM circuit in accordance with the target rotation velocity. The AND circuit receives both the on-signal outputted from the first PWM circuit and the on-signal outputted from the second PWM circuit and outputs a third on-signal which is a logical product of the received on-signals. The rotation velocity of the motor is adjusted to a target rotation velocity by intermittently applying a voltage to the motor in accordance with the on-signal outputted by the AND circuit.
In the controller having two PWM circuits, as in the controller disclosed in the Patent Document 1, a duty ratio of the on-signal outputted by the AND circuit (duty ratio is a ratio of a period during which the on-signal is outputted with respect to a total period) is made to vary by causing the pulse widths of the on-signals outputted by the two PWM circuits to vary respectively. In the controller of the Patent Document 1, for instance, when a detected actual rotation velocity of the motor is less than a predetermined threshold velocity, the pulse width (first pulse width) of the on-signal outputted by the first PWM circuit is fixed to a predetermined width, while the pulse width (second pulse width) of the on-signal outputted by the second PWM circuit is caused to vary. In this case, the AND circuit outputs intermittently the on-signal outputted by the first PWM circuit in synchronization with the on-signal outputted by the second PWM circuit. It becomes possible as a result to further reduce the duty ratio of the on-signal outputted by the AND circuit, with the pulse width of the on-signal outputted by the AND circuit fixed to a predetermined width.
Patent Document 1: Japanese Utility Model Application Publication No.H03-011397